An energy storage device like a lithium-ion secondary battery had been used as a power source of mobile equipment such as a notebook computer or a mobile phone. The energy storage device has been recently applied to wider fields and has been used as a power source of an electric vehicle. Such an energy storage device requires a technique for accurate estimation of discharge capacity (battery capacity) in a certain deterioration state.
There have been conventionally proposed techniques for estimation of discharge capacity of an energy storage device (see JP-A-2000-228227 and H Yoshida et al., Electrochemistry 78 (2010), 482, for example).